1. Field of the Invention
The present invention relates to a distance measuring device that measures the distance to an object.
2. Description of the Related Art
There have been various distance measuring devices developed for measuring the distances to objects, as disclosed in Japanese Unexamined Patent Publication Nos. 10-282216, 2001-33543, and 2003-174368, for example. FIG. 1 illustrates the structure of a distance measuring unit that is a conventional distance measuring device. In the distance measuring unit 500 illustrated in FIG. 1, an impulse generating circuit 506 in a transmission unit 504 receives a transmission pulse from a controller 502, and generates an ultrashort pulse (impulse) through a spreading process. This impulse is transmitted as a transmission wave signal from the transmission antenna 512 via a wideband filter 508 and a wideband amplifier 510. The transmission wave signal is reflected by a distance measurement object 600 and is received by a reception antenna 514. The reflected wave signal is then sent to a despreading circuit 522 via a wideband filter 518 and a wideband low noise amplifier 520. The despreading circuit 522 performs despreading on the reflected wave signal, and then outputs a reception pulse. Based on the time difference between the output of the transmission pulse and the input of the reception pulse, the controller 502 determines the distance L to the distance measurement object 600. With the time difference between the output of the transmission pulse and the input of the reception pulse being t and the speed of light being c, the distance L can be determined by the equationL=c×t/2.
By the above described distance measuring technique, however, the time difference between the output of the transmission pulse and the input of the reception pulse contains delays caused at the wideband filter 508 and the wideband amplifier 510 in the transmission unit 504, the transmission antenna 512, the reception antenna 512, and the wideband filter 518 and the wideband low noise amplifier 520 in the reception unit 516, the time spent for the despreading process by the despreading circuit 522, the time spent for the processing by the controller 502, and the likes. As a result, an error is caused in the distance measurement.
Furthermore, the impulse generated from the transmission pulse is expanded at each of the wideband filter 508, the wideband amplifier 510, and the transmission antenna 512. Because of this, several peaks exist in the reception pulse, and therefore, it is difficult to determine which peak should be considered to be the timing of the input of the reception pulse. Also in this aspect, an error is often caused. In a case where the transmission pulse is the one shown in FIG. 2A, for example, the reception pulse have several peaks as shown in FIG. 2B. Even if the device is designed so that the timing of the input of the reception pulse is a peak with a reception intensity of V2 or higher, the measured distance varies depending on which peak is selected as the input timing among peaks p1 through p4.